Many different methods are used to inactivate harmful microorganisms in the pharmaceutical industry, food processing, medicine, and biotechnology. One method, most often used for liquid substances, is a method used in conventional thermal processing. In this method, the temperature of the liquid is kept elevated for a period of time, and higher temperatures usually required shorter time duration to produce the necessary results. In the food industry, however, this method has an adverse effect on flavor, vitamin, and protein content of the final product.
In the biotech industry, millions of genetically engineered, protein-producing E. coli bacteria are added to a nutrient-rich growth medium for the mass production of therapeutic proteins. After the bacteria synthesize the desired product, they are pumped into a high pressure tank, where they remain for a period of time under extremely high pressure until their cell walls burst open, releasing the contents. In some instances, a successful outcome requires that the process be repeated several times. This method is also used in the production of juices and other food products. The advantage of the high pressure treatment, as compared to the more popular heat treatment, is that this method inactivates the microorganisms with minimal harm to vitamins or flavoring. However, this method has a number of shortcomings, especially in the area of economic feasibility and engineering limitations. Economic feasibility is limited by the high cost of capital investment for the equipment, low productivity, and the high labor cost of batch process. Economic feasibility is further limited by the long process time, 30 minutes to 1 hour, which is required by some applications. Engineering limitations include concerns about the construction of high pressure vessels with a large enough capacity to hold substantial quantities of product.
In another method used to inactivate microorganisms, the liquid substance is first pressurized and then depressurized by transferring the liquid into an area of reduced pressure through one or more constrictions, as shown in U.S. Pat. No. 6,120,732. This method is based on the principle that bacteria cannot withstand sudden pressure change and substantial mechanical friction. However, passage of a substantial quantity of liquid substance through a small orifice with high speed results in overheating of the orifice due to friction, and leads, in liquids such as milk, to the buildup of a hard substance (milk stone) on the tip of the orifice. The formation of such “milk stone” has a negative effect on the process and often can even block the orifice completely. Other problems include limited throughput and the difficulty of maintaining the liquid under high pressure in a vessel, from which a substantial volume of liquid escapes to the low pressure vessel. These problems render this method impractical for the mass production. Finally, since, in the most cases, the percentage of inactivated bacteria is insufficient, additional treatments are often required to achieve acceptable results.
In another method, a special restrictive nozzle is used in place of an orifice. As in the above method, a partial inactivation of the bacteria is achieved by both sudden pressure drop and mechanical friction. In addition, the restrictive nozzle causes the atomization, or break-up, of the liquid substance into tiny particles. The atomized product is then treated with steam vapor. In this treatment, the atomized particles, when coming into contact with vapor, undergo a sudden temperature rise in addition to the sudden pressure drop. The sudden temperature rise further enhances the inactivation of bacteria. In order to keep the maximum temperature of the treated product down, the vapor temperature would need to be no more then 50-60 degrees Celsius. This is achieved by the introduction of vacuum into the system, as shown in the U.S. Pat. No. 6,277,610. This method, however, does not eliminate the “milk stone” problem or the problem of controlling the product temperature after the nozzle. The difference between the temperature of “cold steam” and the temperature of treated substance is often not substantial enough to effectively inactivate the bacteria. To make this process work, the temperature of the steam would have to be raised, which in turn would adversely affect the quality of the final product.